Which has exactly nothing to do with any additive you put in the oil (or lack thereof.)There are alot of variables...but you have to take into account as well my driving habits too (are you guys familiar how I drive a stick shift?). How I anticipate incoming merging traffic and avoid it, to take advantage of continuing momentum, slowing down and leting go of my gas pedal, several hundred feet and being easy on the brakes, free wheeling, etc, and so on..
Its all about the driving techniques. And it plays a huge part, in my cars fuel economy numbers.
Again, thermodynamics (and exactly where energy losses happen in engines generally, and of what magnitude the various sort of losses are, which tells you what is possible) provides a means to mathematically falsify the sort of efficiency gain claim without having to examine the vehicle itself.